1. Field of the Invention
The present invention relates to automatic gain control circuits. More particularly, the present invention relates to a novel and improved automatic gain control circuit capable of independently controlling multiple variable gain amplifier stages while maintaining an estimate of received signal power.
2. Description of the Related Art
In modern communication systems, it is common for a receiver to contain automatic gain control (AGC) circuitry to amplify or attenuate received signals to a desired reference level for further processing by the receiver. An exemplary AGC circuit is described in U.S. Pat. No. 5,099,204, entitled "LINEAR GAIN CONTROL AMPLIFIER", assigned to the assignee of the present invention and incorporated herein by reference. A communication system using such AGC circuits is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated herein by reference. The foregoing system is also described by EIA/TIA Interim Standard IS-95, entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular system" (hereinafter IS-95), incorporated herein by reference.
A mobile station in an IS-95 system, in addition to requiring that incoming signals be gain controlled for further processing, must ensure that its transmitted signals are tightly power controlled so as not to interfere with other mobile stations in the system. Such a power control scheme is described in U.S. Pat. No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated herein by reference. One element in this power control scheme is the use of a measurement of received signal power, and so in contrast to systems where the only requirement of an AGC circuit is to provide incoming signals at the appropriate reference level, an IS-95 AGC circuit must allow for calculation of received signal strength.
Ideally, amplifiers could be constructed which were perfectly linear, at least over some range. Then the amplifier would be characterized by the equation f(x)=k.sub.1 x, where f(x) is the output, x is the input and k.sub.1 is the gain of the amplifier. In reality, amplifiers are not perfectly linear, and that non-linearity introduces distortion into the signal being amplified. Of all the possible input voltages, an amplifier has what is called its "linear" range and its "non-linear" range. The linear range is where the amplifier most closely approximates a linear amplifier. The distortion introduced can be approximated as a third order component. A more realistic characterization of an amplifier is given by the equation f(x)=k.sub.1 x+k.sub.3 x.sup.3. Here k.sub.3 is the gain of the third order component. An amplifier with a smaller value for k.sub.3 will be more linear than an amplifier with a higher value.
One type of distortion introduced by non-linear amplifiers that is particularly troublesome comes from intermodulation terms of two frequencies that are outside the band of interest for a mobile station. An example of this can be seen when an IS-95 system is deployed in close proximity to a narrowband system such as AMPS or GSM. The performance of an amplifier with respect to intermodulation is given by its IP3 point. For calculation purposes, it is assumed that the transmitters of the desired band and the source of the undesired frequencies are co-located. This means that as a mobile station moves toward the transmitter, both the desired received power and the intermodulation power increase. The IP3 point is the point where the third order intermodulation power of two equal power tones offset in frequency is equal to the desired first order term. To optimize the IP3 performance of an amplifier, the third order gain, k.sub.3, should be minimized.
One way to increase IP3 performance is to increase the "linear" range of the amplifier. Supplying more current to the amplifier can do this. However, in typical mobile communication systems, power in a mobile station is at a premium and increasing current is only done when absolutely necessary. Reduced power consumption translates into increased standby and talk time in a mobile station, or alternatively in a reduced battery requirement that leads to smaller and lighter mobile stations. An alternative to increasing the linear range is to reduce the amplitude of the incoming signal so that it stays within the existing linear range of the amplifier.
IS-95 specifies a minimum level of what it describes as intermod rejection. FIG. 1 shows a typical intermod rejection ratio plot. For a given range of received power, the receiver must be able to tolerate a certain amount of interference, or have a certain intermod rejection ratio (IMR), as shown by the line labeled "spec" between specification points S1 and S2. The intermod rejection ratio of an amplifier with fixed IP3 will increase 1/3 of a dB for every dB increase in received input power. The slope of the spec line may not be 1/3 of a dB per dB, and in fact it is not in IS-95. The IS-95 slope is approximately a 1 dB per dB slope. For a spec as shown, an amplifier must meet the specification at point S2. This would yield an IMR given by the line A1. To meet the specification requirement at point S1, a lower current amplifier could be used which would yield an IMR given by A2. As shown, the amplifier that meets point S2 is overdesigned for point S1. This overdesign can equates to an increase in bias current resulting in reduced battery life, or more expensive components being required, or both.
An AGC design that could exhibit the properties attributed to lines A1 or A2 is shown in FIG. 2. Received signals at antenna 100 are directed to ultra high frequency (UHF) low noise amplifier (LNA) 110. A dashed arrow is shown through amplifier 110 to indicate the option of having it be a variable gain amplifier. That variable gain configuration will be discussed below. The received signal is amplified by LNA 110 and downconverted in mixer 115 via UHF frequency generated by UHF local oscillator 120. The downconverted signal is passed through band pass filter 130 and amplified by intermediate frequency (IF) variable gain amplifier 140. This amplified IF signal is then downconverted in mixer 145 via IF frequency generated by IF frequency generator 150. The received signal is now at baseband, and received signal strength indicator (RSSI) 160 generates an estimate of the received signal power. The difference of this estimate and a reference power stored in power reference 165 is calculated in adder 170, and RX AGC 180 acts on this error difference to produce the appropriate AGC_VALUE 195. AGC_VALUE 195 is fed through a linearizer 190 to variable gain amplifier 140. Linearizer 190 compensates for any non-linear dB/V characteristics of variable gain amplifier 140. Linearization is described in U.S. Pat. No. 5,627,857, entitled "LINEARIZED DIGITAL AUTOMATIC GAIN CONTROL", assigned to the assignee of the present invention and incorporated herein by reference. RX AGC 180 could be a variety of circuits as known in the art which alter AGC_VALUE so as to drive the difference calculated in adder 170 to as close to zero as possible. Once this loop is converged, the baseband signal out of mixer 145 is at the appropriate input power level and can be further demodulated (in circuitry not shown). Typically, the IF downconversion is done on the in-phase and quadrature components of the signal, and additional filtering is performed, but those details are not shown for the sake of clarity. The circuit as just described will exhibit the IMR response of line A1 in FIG. 1 when designed at a fixed current level. Note that AGC_VALUE can be used to estimate the received power, but only after factoring in the gain from the entire receive chain.
One way to reduce the overuse of current is to use a lower current amplifier than would be required to generate line A1 and introduce variability in the front end gain stage, LNA 110, as shown by the dashed arrow in FIG. 2. As an example, assume that this is a switched fixed gain LNA, meaning it is either on with a fixed gain or bypassed altogether. When LNA 110 is switched out, the amplification will be reduced, or attenuation will be added. This reduces the linear range requirement for IF amplifier 140.
When the LNA stage is switched out, a performance cost is paid through an increased noise floor. The carrier to interference (C/I) figure is approximated as this thermal noise floor from the amplifier circuits plus the intermod components plus the co-channel interference. The performance of the demodulator is a function of the baseband C/I. As the received power (C) increases, the total interference can increase. Given that the noise floor remains approximately constant if no LNA switching is performed, the excess margin can be traded for improved IP3 performance by switching off the amplifier which results in improved IP3 performance at the expense of increased noise floor.
FIG. 3 shows an IMR spec that is the same as shown in FIG. 1. However, the IMR of the variable gain LNA just described is quite different from IMR lines A1 or A2. The amplifier must have a linear range and current consumption to support the IMR given by line segment R1. This is less current than the amplifier needed to supply line A1 of FIG. 1. As the input power is increased, the range of linearity of this amplifier is used up, and without more, would fall below spec point S3. Instead, the attenuation is added by switching out LNA 110 and therefore the inputs to IF amplifier 140 are back within its linear range and the IP3 performance goes up, in this example to the performance comparable to the amplifier required to produce line A1. This is shown by line segment R2. In a similar manner, if a truly variable gain LNA 110 is used, rather than the simple on/off example just demonstrated, the performance of the AGC amplifier chain can be made very close to the minimum spec required, and hence the minimum power consumption required.
The total gain value in the amplifier chain in the AGC can be used as a measure of the total received power. This is because the basic function of an AGC is to take an input power level and reduce it to a reference power level by applying a gain factor. If the gain factor is known, then the actual received power is also known since the reference power is known. However, for improved IP3 performance, it is desirable to be able to change the distribution of attenuation or gain throughout the amplifiers in the AGC chain. But note that once the gain is distributed among stages, AGC_VALUE 195 is no longer a good estimate. As shown above, a distribution of gain through a variety of amplifiers does not necessarily yield automatically an overall AGC gain value which can be used as a received power estimate (and hence for a transmit power estimate). There is a need in the art for an AGC circuit that is controllable to enhance IP3 performance while yielding a usable estimate of the received power.